This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Primary Objectives 1. To evaluate the relative potencies of two monotherapy regimens (Tenofovir(TDF) alone vs. Abacavir(ABC) alone) compared to a dual NRTI regimen of TDF + ABC as assessed by the short-term virologic response (slope of the phase I viral decay and change in HIV RNA at day 7). 2. To compare the plasma and intracellular PK data of the two monotherapy regimens compared to the dual NRTI regimen. Secondary Objectives 1. To evaluate the change in cellular regulatory enzymes involved with nucleoside analogue transport across cell membranes as assessed by RT-PCR of specific mRNA transcripts after 21 days of TDF+ABC exposure. 2. To determine if NRTI-associated mutations emerge after 7 days of ABC or TDF monotherapy or after 7 days of dual NRTI therapy with ABC + TDF. 3. To compare the relative viral potency of TDF monotherapy versus ABC monotherapy. 4. To evaluate the long-term viral response to Efavirenz (EFV) + ABC + 3TC after two 7-day sequences of mono/dual-therapy. 5. To evaluate the relative toxicities of the two monotherapy treatment regimens.